I decided to see how well ZFS copies=n would stand up to on-disk corruption today. Spoiler alert: not great.

First step, I created a 1GB virtual disk, made a zpool out of it with 8K blocks, and set copies=2.

me@locutus:~$ sudo qemu-img create -f qcow2 /data/test/copies/0.qcow2 1G
me@locutus:~$ sudo qemu-nbd -c /dev/nbd0 /data/test/copies/0.qcow2 1G
me@locutus:~$ sudo zpool create -oashift=12 test /data/test/copies/0.qcow2
me@locutus:~$ sudo zfs set copies=2 test

Now, I wrote 400 1MB files to it – just enough to make the pool roughly 85% full, including the overhead due to copies=2.

me@locutus:~$ cat /tmp/makefiles.pl
#!/usr/bin/perl

for ($x=0; $x<400 ; $x++) {
	print "dd if=/dev/zero bs=1M count=1 of=$x\n";
	print `dd if=/dev/zero bs=1M count=1 of=$x`;
}

With the files written, I backed up my virtual filesystem, fully populated, so I can repeat the experiment later.

me@locutus:~$ sudo zpool export test
me@locutus:~$ sudo cp /data/test/copies/0.qcow2 /data/test/copies/0.qcow2.bak
me@locutus:~$ sudo zpool import test

Now, I write corrupt blocks to 10% of the filesystem. (Roughly: it's possible that the same block was overwritten with a garbage block more than once.) Note that I used a specific seed, so that I can recreate the scenario exactly, for more runs later.

me@locutus:~$ cat /tmp/corruptor.pl
#!/usr/bin/perl

# total number of blocks in the test filesystem
$numblocks=131072;

# desired percentage corrupt blocks
$percentcorrupt=.1;

# so we write this many corrupt blocks
$corruptloop=$numblocks*$percentcorrupt;

# consistent results for testing
srand 32767;

# generate 8K of garbage data
for ($x=0; $x<8*1024; $x++) {
	$garbage .= chr(int(rand(256)));
}

open FH, "> /dev/nbd0";

for ($x=0; $x<$corruptloop; $x++) {
	$blocknum = int(rand($numblocks-100));
	print "Writing garbage data to block $blocknum\n";
	seek FH, ($blocknum*8*1024), 0;
	print FH $garbage;
}

close FH;

Okay. When I scrub the filesystem I just wrote those 10,000 or so corrupt blocks to, what happens?

me@locutus:~$ sudo zpool scrub test ; sudo zpool status test
  pool: test
 state: ONLINE
status: One or more devices has experienced an error resulting in data
	corruption.  Applications may be affected.
action: Restore the file in question if possible.  Otherwise restore the
	entire pool from backup.
   see: http://zfsonlinux.org/msg/ZFS-8000-8A
  scan: scrub repaired 133M in 0h1m with 1989 errors on Mon May  9 15:56:11 2016
config:

	NAME        STATE     READ WRITE CKSUM
	test        ONLINE       0     0 1.94K
	  nbd0      ONLINE       0     0 8.94K

errors: 1989 data errors, use '-v' for a list
me@locutus:~$ sudo zpool status -v test | grep /test/ | wc -l
385

OUCH. 385 of my 400 total files were still corrupt after the scrub! Copies=2 didn't do a great job for me here. 🙁

What if I try it again, this time just writing garbage to 1% of the blocks on disk, not 10%? First, let's restore that backup I cleverly made:

root@locutus:/data/test/copies# zpool export test
root@locutus:/data/test/copies# qemu-nbd -d /dev/nbd0
/dev/nbd0 disconnected
root@locutus:/data/test/copies# pv < 0.qcow2.bak > 0.qcow2
 999MB 0:00:00 [1.63GB/s] [==================================>] 100%            
root@locutus:/data/test/copies# qemu-nbd -c /dev/nbd0 /data/test/copies/0.qcow2
root@locutus:/data/test/copies# zpool import test
root@locutus:/data/test/copies# zpool status test | tail -n 5
	NAME        STATE     READ WRITE CKSUM
	test        ONLINE       0     0     0
	  nbd0      ONLINE       0     0     0

errors: No known data errors

Alright, now let's change $percentcorrupt from 0.1 to 0.01, and try again. How'd we do after only corrupting 1% of the blocks on disk?

root@locutus:/data/test/copies# zpool status test
  pool: test
 state: ONLINE
status: One or more devices has experienced an error resulting in data
	corruption.  Applications may be affected.
action: Restore the file in question if possible.  Otherwise restore the
	entire pool from backup.
   see: http://zfsonlinux.org/msg/ZFS-8000-8A
  scan: scrub repaired 101M in 0h0m with 72 errors on Mon May  9 16:13:49 2016
config:

	NAME        STATE     READ WRITE CKSUM
	test        ONLINE       0     0    72
	  nbd0      ONLINE       0     0 1.08K

errors: 64 data errors, use '-v' for a list
root@locutus:/data/test/copies# zpool status test -v | grep /test/ | wc -l
64

Still not great. We lost 64 out of our 400 files. Tenth of a percent?

root@locutus:/data/test/copies# zpool status -v test
  pool: test
 state: ONLINE
status: One or more devices has experienced an error resulting in data
	corruption.  Applications may be affected.
action: Restore the file in question if possible.  Otherwise restore the
	entire pool from backup.
   see: http://zfsonlinux.org/msg/ZFS-8000-8A
  scan: scrub repaired 12.1M in 0h0m with 2 errors on Mon May  9 16:22:30 2016
config:

	NAME        STATE     READ WRITE CKSUM
	test        ONLINE       0     0     2
	  nbd0      ONLINE       0     0   105

errors: Permanent errors have been detected in the following files:

        /test/300
        /test/371

Damn. We still lost two files, even with only writing 130 or so corrupt blocks. (The missing 26 corrupt blocks weren't picked up by the scrub because they happened in the 15% or so of unused space on the pool, presumably: a scrub won't check unused blocks.) OK, what if we try a control - how about we corrupt the same tenth of a percent of the filesystem (105 blocks or so), this time without copies=2 set? To make it fair, I wrote 800 1MB files to the same filesystem this time using the default copies=1 - this is more files, but it's the same percentage of the filesystem full. (Interestingly, this went a LOT faster. Perceptibly, more than twice as fast, I think, although I didn't actually time it.)

Now with our still 84% full /test zpool, but this time with copies=1, I corrupted the same 0.1% of the total block count.

root@locutus:/data/test/copies# zpool status test
  pool: test
 state: ONLINE
status: One or more devices has experienced an error resulting in data
	corruption.  Applications may be affected.
action: Restore the file in question if possible.  Otherwise restore the
	entire pool from backup.
   see: http://zfsonlinux.org/msg/ZFS-8000-8A
  scan: scrub repaired 8K in 0h0m with 98 errors on Mon May  9 16:28:26 2016
config:

	NAME                         STATE     READ WRITE CKSUM
	test                         ONLINE       0     0    98
	  /data/test/copies/0.qcow2  ONLINE       0     0   198

errors: 93 data errors, use '-v' for a list

Without copies=2 set, we lost 93 files instead of 2. So copies=n was definitely better than nothing for our test of writing 0.1% of the filesystem as bad blocks... but it wasn't fabulous, either, and it fell flat on its face with 1% or 10% of the filesystem corrupted. By comparison, a truly redundant pool - one with two disks in it, in a mirror vdev - would have survived the same test (corrupting ANY number of blocks on a single disk) with flying colors.

The TL;DR here is that copies=n is better than nothing... but not by a long shot, and you do give up a lot of performance for it. Conclusion: play with it if you like, but limit it only to extremely important data, and don't make the mistake of thinking it's any substitute for device redundancy, much less backups.

So ZFS dedup is a complete lose. What about compression?

Compression is a hands-down win. LZ4 compression should be on by default for nearly anything you ever set up under ZFS. I typically have LZ4 on even for datasets that will house database binaries… yes, really. Let’s look at two quick test runs, on a Xeon E3 server with 32GB ECC RAM and a pair of Samsung 850 EVO 1TB disks set up as a mirror vdev.

This is an inline compression torture test: we’re reading pseudorandom data (completely incompressible) and writing it to an LZ4 compressed dataset.

root@lab:/data# pv < in.rnd > incompressible/out.rnd
7.81GB 0:00:22 [ 359MB/s] [==================================>] 100%

root@lab:/data# zfs get compressratio data/incompressible
NAME                 PROPERTY       VALUE  SOURCE
data/incompressible  compressratio  1.00x  -

359MB/sec write… yyyyyeah, I’d say LZ4 isn’t hurting us too terribly here – and this is a worst case scenario. What about something a little more realistic? Let’s try again, this time with a raw binary of my Windows Server 2012 R2 “gold” image (the OS is installed and Windows Updates are applied, but nothing else is done to it):

root@lab:/data/test# pv < win2012r2-gold.raw > realworld/win2012r2-gold.out
8.87GB 0:00:17 [ 515MB/s] [==================================>] 100%

Oh yeah – 515MB/sec this time. Definitely not hurting from using our LZ4 compression. What’d we score for a compression ratio?

root@lab:/data# zfs get compressratio data/realworld
NAME            PROPERTY       VALUE  SOURCE
data/realworld  compressratio  1.48x  -

1.48x sounds pretty good! Can we see some real numbers on that?

root@lab:/data# ls -lh /data/realworld/win2012r2-gold.raw
-rw-rw-r-- 1 root root 8.9G Feb 24 18:01 win2012r2-gold.raw
root@lab:/data# du -hs /data/realworld
6.2G	/data/realworld

8.9G of data in 6.2G of space… with sustained writes of 515MB/sec.

What if we took our original 8G of incompressible data, and wrote it to an uncompressed dataset?

root@lab:/data#  zfs create data/uncompressed
root@lab:/data# zfs set compression=off data/uncompressed
root@lab:/data# cat 8G.in > /dev/null ; # this is to make sure our source data is preloaded in the ARC
root@lab:/data# pv < 8G.in > uncompressed/8G.out
7.81GB 0:00:21 [ 378MB/s] [==================================>] 100% 

So, our worst case scenario – completely incompressible data – means a 5% performance hit, and a more real-world-ish scenario – copying a Windows Server installation – means a 27% performance increase. That’s on fast solid state, of course; the performance numbers will look even better on slower storage (read: spinning rust), where even worst-case writes are unlikely to slow down at all.

Yep, that’s a win.

Even if you have the RAM for it (and we’re talking a good 6GB or so per TB of storage), ZFS deduplication is, unfortunately, almost certainly a lose.

I don’t usually have that much RAM to spare, but one server has 192GB of RAM and only a few terabytes of storage – and it stores a lot of VM images, with obvious serious block-level duplication between images. Dedup shows at 1.35+ on all the datasets, and would be higher if one VM didn’t have a couple of terabytes of almost dup-free data on it.

That server’s been running for a few years now, and nobody using it has complained. But I was doing some maintenance on it today, splitting up VMs into their own datasets, and saw some truly abysmal performance.

root@virt0:/data/images# pv < jabberserver.qcow2 > jabber/jabberserver.qcow2 206MB 0:00:31 [7.14MB/s] [>                  ]  1% ETA 0:48:41

7MB/sec? UGH! And that’s not even a sustained average; that’s just where it happened to be when I killed the process. This server should be able to sustain MUCH better performance than that, even though it’s reading and writing from the same pool. So I checked, and saw that dedup was on:

root@virt0:~# zpool list
NAME   SIZE  ALLOC   FREE    CAP  DEDUP  HEALTH  ALTROOT
data  7.06T  2.52T  4.55T    35%  1.35x  ONLINE  -

In theory, you’d think that dedup would help tremendously with exactly this operation: copying a quiesced VM from one dataset to another on the same pool. There’s no need for a single block of data to be rewritten, just more pointers added to the metadata for the existing blocks. However, dedup looked like the obvious culprit for my performance woes here, so I disabled it and tried again:

root@virt0:/data/images# pv < jabberserver.qcow2 > jabber/jabberserver.qcow219.2GB 0:04:58 [65.7MB/s] [============>] 100%

Yep, that’s more like it.

TL;DR: ZFS dedup sounds like a great idea, but in the real world, it sucks. Even on a machine built to handle it. Even on exactly the kind of storage (a bunch of VMs with similar or identical operating systems) that seems tailor-made for it. I do not recommend its use for pretty much any conceivable workload.

(On the other hand, LZ4 compression is an unqualified win.)

This comes up far too often, so rather than continuing to explain it over and over again, I’m going to try to do a really good job of it once and link to it here.

What’s ECC RAM? Is it a good idea?

The ECC stands for Error Correcting Checksum. In a nutshell, ECC RAM is a special kind of server-grade memory that can detect and repair some of the most common kinds of in-memory corruption. For more detail on how ECC RAM does this, and which types of errors it can and cannot correct, the rabbit hole’s over here.

Now that we know what ECC RAM is, is it a good idea? Absolutely. In-memory errors, whether due to faults in the hardware or to the impact of cosmic radiation (yes, really) are a thing. They do happen. And if it happens in a particularly strategic place, you will lose data to it. Period. There’s no arguing this.

What’s ZFS? Is it a good idea?

ZFS is, among other things, a checksumming filesystem. This means that for every block committed to storage, a strong hash (somewhat misleadingly AKA checksum) for the contents of that block is also written. (The validation hash is written in the pointer to the block itself, which is also checksummed in the pointer leading to itself, and so on and so forth. It’s turtles all the way down. Rabbit hole begins over here for this one.)

Is this a good idea? Absolutely. Combine ZFS checksumming with redundancy or parity, and now you have a self-healing array. If a block is corrupt on disk, the next time it’s read, ZFS will see that it doesn’t match its checksum and will load a redundant copy (in the case of mirror vdevs or multiple copy storage) or rebuild a parity copy (in the case of RAIDZ vdevs), and assuming that copy of the block matches its checksum, will silently feed you the correct copy instead, and log a checksum error against the first block that didn’t pass.

ZFS also supports scrubs, which will become important in the next section. When you tell ZFS to scrub storage, it reads every block that it knows about – including redundant copies – and checks them versus their checksums. Any failing blocks are automatically overwritten with good blocks, assuming that a good (passing) copy exists, either redundant or as reconstructed from parity. Regular scrubs are a significant part of maintaining a ZFS storage pool against long term corruption.

Is ZFS and non-ECC worse than not-ZFS and non-ECC? What about the Scrub of Death?

OK, it’s pretty easy to demonstrate that a flipped bit in RAM means data corruption: if you write that flipped bit back out to disk, congrats, you just wrote bad data. There’s no arguing that. The real issue here isn’t whether ECC is good to have, it’s whether non-ECC is particularly problematic with ZFS. The scenario usually thrown out is the the much-dreaded Scrub Of Death.

TL;DR version of the scenario: ZFS is on a system with non-ECC RAM that has a stuck bit, its user initiates a scrub, and as a result of in-memory corruption good blocks fail checksum tests and are overwritten with corrupt data, thus instantly murdering an entire pool. As far as I can tell, this idea originates with a very prolific user on the FreeNAS forums named Cyberjock, and he lays it out in this thread here. It’s a scary idea – what if the very thing that’s supposed to keep your system safe kills it? A scrub gone mad! Nooooooo!

The problem is, the scenario as written doesn’t actually make sense. For one thing, even if you have a particular address in RAM with a stuck bit, you aren’t going to have your entire filesystem run through that address. That’s not how memory management works, and if it were how memory management works, you wouldn’t even have managed to boot the system: it would have crashed and burned horribly when it failed to load the operating system in the first place. So no, you might corrupt a block here and there, but you’re not going to wring the entire filesystem through a shredder block by precious block.

But we’re being cheap here. Say you only corrupt one block in 5,000 this way. That would still be hellacious. So let’s examine the more reasonable idea of corrupting some data due to bad RAM during a scrub. And let’s assume that we have RAM that not only isn’t working 100% properly, but is actively goddamn evil and trying its naive but enthusiastic best to specifically kill your data during a scrub:

First, you read a block. This block is good. It is perfectly good data written to a perfectly good disk with a perfectly matching checksum. But that block is read into evil RAM, and the evil RAM flips some bits. Perhaps those bits are in the data itself, or perhaps those bits are in the checksum. Either way, your perfectly good block now does not appear to match its checksum, and since we’re scrubbing, ZFS will attempt to actually repair the “bad” block on disk. Uh-oh! What now?

Next, you read a copy of the same block – this copy might be a redundant copy, or it might be reconstructed from parity, depending on your topology. The redundant copy is easy to visualize – you literally stored another copy of the block on another disk. Now, if your evil RAM leaves this block alone, ZFS will see that the second copy matches its checksum, and so it will overwrite the first block with the same data it had originally – no data was lost here, just a few wasted disk cycles. OK. But what if your evil RAM flips a bit in the second copy? Since it doesn’t match the checksum either, ZFS doesn’t overwrite anything. It logs an unrecoverable data error for that block, and leaves both copies untouched on disk. No data has been corrupted. A later scrub will attempt to read all copies of that block and validate them just as though the error had never happened, and if this time either copy passes, the error will be cleared and the block will be marked valid again (with any copies that don’t pass validation being overwritten from the one that did).

Well, huh. That doesn’t sound so bad. So what does your evil RAM need to do in order to actually overwrite your good data with corrupt data during a scrub? Well, first it needs to flip some bits during the initial read of every block that it wants to corrupt. Then, on the second read of a copy of the block from parity or redundancy, it needs to not only flip bits, it needs to flip them in such a way that you get a hash collision. In other words, random bit-flipping won’t do – you need some bit flipping in the data (with or without some more bit-flipping in the checksum) that adds up to the corrupt data correctly hashing to the value in the checksum. By default, ZFS uses 256-bit SHA validation hashes, which means that a single bit-flip has a 1 in 2^256 chance of giving you a corrupt block which now matches its checksum. To be fair, we’re using evil RAM here, so it’s probably going to do lots of experimenting, and it will try flipping bits in both the data and the checksum itself, and it will do so multiple times for any single block. However, that’s multiple 1 in 2^256 (aka roughly 1 in 10^77) chances, which still makes it vanishingly unlikely to actually happen… and if your RAM is that damn evil, it’s going to kill your data whether you’re using ZFS or not.

But what if I’m not scrubbing?

Well, if you aren’t scrubbing, then your evil RAM will have to wait for you to actually write to the blocks in question before it can corrupt them. Fortunately for it, though, you write to storage pretty much all day long… including to the metadata that organizes the whole kit and kaboodle. First time you update the directory that your files are contained in, BAM! It’s gotcha! If you stop and think about it, in this evil RAM scenario ZFS is incredibly helpful, because your RAM now needs to not only be evil but be bright enough to consistently pull off collision attacks. So if you’re running non-ECC RAM that turns out to be appallingly, Lovecraftianishly evil, ZFS will mitigate the damage, not amplify it.

If you are using ZFS and you aren’t scrubbing, by the way, you’re setting yourself up for long term failure. If you have on-disk corruption, a scrub can fix it only as long as you really do have a redundant or parity copy of the corrupted block which is good. Once you corrupt all copies of a given block, it’s too late to repair it – it’s gone. Don’t be afraid of scrubbing. (Well, maybe be a little wary of the performance impact of scrubbing during high demand times. But don’t be worried about scrubbing killing your data.)

I’ve constructed a doomsday scenario featuring RAM evil enough to kill my data after all! Mwahahaha!

OK. But would using any other filesystem that isn’t ZFS have protected that data? ‘Cause remember, nobody’s arguing that you can lose data to evil RAM – the argument is about whether evil RAM is more dangerous with ZFS than it would be without it.

I really, really want to use the Scrub Of Death in a movie or TV show. How can I make it happen?

What you need here isn’t evil RAM, but an evil disk controller. Have it flip one bit per block read or written from disk B, but leave the data from disk A alone. Now scrub – every block on disk B will be overwritten with a copy from disk A, but the evil controller will flip bits on write, so now, all of disk B is written with garbage blocks. Now start flipping bits on write to disk A, and it will be an unrecoverable wreck pretty quickly, since there’s no parity or redundancy left for any block. Your choice here is whether to ignore the metadata for as long as possible, giving you the chance to overwrite as many actual data blocks as you can before the jig is up as they are written to by the system, or whether to pounce straight on the metadata and render the entire vdev unusable in seconds – but leave the actual data blocks intact for possible forensic recovery.

Alternately you could just skip straight to step B and start flipping bits as data is written on any or all individual devices, and you’ll produce real data loss quickly enough. But you specifically wanted a scrub of death, not just bad hardware, right?

I don’t care about your logic! I wish to appeal to authority!

OK. “Authority” in this case doesn’t get much better than Matthew Ahrens, one of the cofounders of ZFS at Sun Microsystems and current ZFS developer at Delphix. In the comments to one of my filesystem articles on Ars Technica, Matthew said “There’s nothing special about ZFS that requires/encourages the use of ECC RAM more so than any other filesystem.”

Hope that helps. =)